This is the maximum size of a shared memory segment (ie the largest value that can be used by shmget). Its theoretical maximum value is 4294967295 (4GB), but practical considerations usually limit it to less than this. There is no reason not to tune this value as high as possible, since no kernel resources are allocated based on this parameter. Solaris 10 sets shmmax to 1/4 physical memory by default, vs 512k for previous versions.